One was a planned, intelligence-led operation; the other a full-scale security alert that raised fleeting fears a lone wolf extremist might be acting out a terror plot on a passenger coach bound for London.

In the space of a few hours, the police and security services were put through the two extremes of counterterrorist engagement that are likely to dominate their work in the runup to the Olympic Games.

On the M6 toll road, near Lichfield, Staffordshire, the kind of public alertness that anti-terrorist investigators encourage led to the evacuation of 48 passengers from a coach and the closure of the motorway for seven hours, when a concerned passenger apparently mistook a fake, electronic cigarette that gave off a vapour for a bomb.

A few hours earlier in the Olympic area of Stratford, east London, three brothers were among six people arrested by counter-terrorist officers and MI5, in a planned operation, following weeks of surveillance.

The arrests were not initiated as a result of any imminent attack but after counter-terror investigators weighed up the evidence they had gathered so far against the suspects, and the perceived risks of leaving them at liberty for longer with the Olympic Games on the horizon.

One of the brothers, a 24-year-old, was hit with a Taser when he was arrested with his siblings at their home.

Scotland Yard confirmed that one of the three siblings had worked as a community support officer for the Metropolitan police between May 2007 and September 2009.

In Ealing, west London, Richard Dart, also known as Salahuddin al-Britani, a Muslim convert from Dorset, was seized off the street by police as part of the same operation.

The activities in the north and south of England in one day were a sign that – with hundreds of counter-terror investigations known to be live – the risk assessments guiding when to intervene are changing as the Olympic Games draw closer.

This "collapsing timescale", as investigators call it, is likely to result in increasing numbers of arrests and visible disruptive activity in the coming days, the Guardian understands.

The police and security service are also preparing for a deluge of intelligence from foreign police forces and intelligence agencies, who will pass on every piece of information, no matter how seemingly insignificant, to ensure they are not sitting on evidence of a potential threat to the Games.

This combined with heightened public awareness, as exhibited on the M6 , means those working in counter-terrorism face being tested to the limit over the coming days and weeks.

But as Scotland Yard officers began questioning the six individuals arrested in London on Thursday, they were issued with an unequivocal warning from the government's terror watchdog not to let any understandable jumpiness override legal imperatives.

David Anderson QC, the independent reviewer of terror laws, said the police must have reasonable suspicion before they move in to make arrests.

"We have a lot of people in intelligence agencies manning their desks, having their leave cancelled, and no doubt there will be a temptation for people to use that time as the Olympics become closer to arrest people," he said in an interview with Muslim News. "There is a possibility that people will get worried and they will resort to power of arrest. I am watching like a hawk."

In Stratford neighbours were woken by a succession of bangs around 4am, and looked out to see around 60 police, including armed officers, raid a property in Abbey Road.

Three brothers, aged, 18, 24, and 26, were arrested in the operation.

Neighbour Stephen Macguire, 23, said the blasts shook his house. "It was just the loudest explosion.

"I looked up and thought Canary Wharf was gone. I [got] out of bed and nearly had a heart attack. I could see ... endless smoke and torches point up at the house's windows like they were looking for someone."

Another neighbour, who did not want to be identified, said: "I looked out of the window ... and all the street lights had been turned off. There were no sirens. Around 4.10am I heard the loud bang. I literally saw it. They used something to blow the door in, it was some kind of explosion."

The neighbour, who lives in a block of flats opposite the property, added that the same house had been raided last year.

In Ealing, west London, sources confirmed that Dart was arrested off the street by armed officers.

A 21-year-old man was arrested from a residential address in west London, and a 30-year-old woman from a separate address. All six individuals – who can be held for up to 14 days – are being questioned on suspicion of the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000.

In Staffordshire the police defended their response to the alert by a passenger on board a Megabus coach run by Stagecoach, which was on its way from Preston to Victoria station in London.

The alarm was raised when a passenger saw an individual, described as of Asian appearance, pouring what they believed was liquid into a bag that was emitting vapours, the police said. The passenger alerted the driver, who called the police around 8.20am.

A full multi-agency response was put into motion, with an army bomb disposal team, a chemical and biological weapons team, armed police, counter-terrorism officers, ambulance and fire crews, called to the scene at Weeford, near Lichfield. Passengers were evacuated one by one from the coach. Drivers caught in the traffic jam as emergency services blocked the road in front of them were told by police to shut their car windows and not to use the air conditioning.

Nick Jones, a motorist whose car was stopped on the motorway, told BBC News: "I was beginning to feel a little uneasy. I was beginning to look around for an escape route."

By midday the police said the incident was not terror-related and no crime had been committed.

However, the northbound carriageway of Britain's only toll motorway was shut for nearly five hours and the southbound for seven hours.

After arriving at Victoria coach station in London on a replacement bus last night, one of the passengers, student Vermilion Von Kangur, said she had feared for her life as armed police swooped around the coach. "Some people thought there was a bomb on the bus, some people thought it was going to explode," the 20-year-old said. "Some people thought the bus had broken down, or that there was a prisoner on board. I was scared."

She said everyone on board was made to leave it one by one at 10-minute intervals with their hands visible, as officers looked on with their guns poised. "My legs were like jelly, I couldn't walk," she said. "I felt very intimidated. I thought if I moved I would get shot."

She said passengers were not given any information about what had happened after the coach pulled over without warning at around 8.30am. She said: "The driver was talking to police but they weren't saying anything to us. People thought they were going to die. They were calling their families.

"People didn't know what was going on so we only had to use our imaginations and picture the worst."

Staffordshire police said they acted after a "genuinely concerned" member of the public mistook the vapour from an electronic cigarette – used by smokers to circumvent the ban on smoking on public transport – for something more sinister.

"Given the nature of the report we responded swiftly and proportionately, treating the information as credible and extremely seriously," a spokeswoman for the force said. "Our utmost priority was the safety and security of those people on the coach and those travelling on the motorway."

The activity came after the arrest last week of two men in east London when a member of the public raised fears about a security threat to the Olympic canoeing venue on the river Lea.

Police arrested the men, aged 18 and 32, last Thursday morning on suspicion of the commission, preparation or instigation of acts of terrorism after a major operation along the banks of the Olympic canoeing venue in Waltham Abbey, Essex. The men were held for two days before being released on Saturday with no further action taken.

The convert

A British Muslim convert who appeared in a BBC documentary was reportedly one of six people arrested on Thursday over a suspected terror plot. Three live just over a mile from the Olympic site. Richard Dart, 29, who changed his name to Salahuddin al-Britani, appeared in a BBC3 documentary, My Brother the Islamist, made by his stepbrother Robb Leech last year. The programme told how Dart, originally from Weymouth, Dorset, had converted to Islam and he spoke of his understanding of his new religion.

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